Error concealment method and apparatus for audio signal and decoding method and apparatus for audio signal using the same

ABSTRACT

An error concealment method and apparatus for an audio signal and a decoding method and apparatus for an audio signal using the error concealment method and apparatus. The error concealment method includes selecting one of an error concealment in a frequency domain and an error concealment in a time domain as an error concealment scheme for a current frame based on a predetermined criteria when an error occurs in the current frame, selecting one of a repetition scheme and an interpolation scheme in the frequency domain as the error concealment scheme for the current frame based on a predetermined criteria when the error concealment in the frequency domain is selected, and concealing the error of the current frame using the selected scheme.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of prior application Ser.No. 14/589,617, filed on Jan. 5, 2015, which is a continuationapplication of prior application Ser. No. 13/933,247, filed on Jul. 2,2013, now U.S. Pat. No. 9,373,331, which is a continuation applicationof Ser. No. 13/544,203, filed on Jul. 9, 2012, now U.S. Pat. No.8,676,569, which is a continuation application of Ser. No. 11/930,752,filed on Oct. 31, 2007, now U.S. Pat. No. 8,219,393, in the UnitedStates Patent and Trademark Office, which claims priority under 35U.S.C. § 119(a) from Korean Patent Application Nos. 10-2006-0117252filed on Nov. 24, 2006, and 10-2006-0128945 filed on Dec. 15, 2006, inthe Korean Intellectual Property Office, the disclosures of which areincorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present general inventive concept relates to a method of andapparatus to conceal an error of an audio signal, and more particularly,to an error concealment method and apparatus for an audio signal, inwhich modulation noise can be prevented from being generated when anerror occurring over a long interval is concealed by a repetition schemein a frequency domain.

2. Description of the Related Art

An error may occur during transmission of an encoded audio signal over awired or wireless network such as a terrestrial digital multimediabroadcasting (T-DMB) network or an Internet protocol (IP) network.Without proper processing of the error, annoying distortion would occurdue to a transmission error, resulting in degradation of sound quality.

Conventionally, in order to conceal the error of an audio signal, arepetition scheme that reconstructs an error frame by repeating thespectrum of a previous good frame or an interpolation scheme thatreconstructs an error frame by interpolating the spectrum of a previousgood frame and the spectrum of a next good frame were used.

However, when the error occurs over a long interval, the same spectrumof the previous good frame is repeated over the long interval, causingmodulation noise.

SUMMARY OF THE INVENTION

The present general inventive concept provides an error concealmentmethod and apparatus for an audio signal, in which a repetition schemeand an interpolation scheme in a frequency domain are selectivelyutilized to conceal an error that occurs in a frame and an errorconcealment in a time domain is used when it is difficult or impossibleto conceal the error in the frequency domain, thereby preventing soundquality from degrading due to modulation noise.

The present general inventive concept also provides a decoding methodand apparatus for an audio signal, in which sound quality can beprevented from degrading due to modulation noise by reconstructing anerror frame using the error concealment method and apparatus, therebyimproving perceptual sound quality.

The present general inventive concept also provides a computer-readablemedium having recorded thereon a program to implement one of the errorconcealment method and the decoding method.

Additional aspects and utilities of the present general inventiveconcept will be set forth in part in the description which follows and,in part, will be obvious from the description, or may be learned bypractice of the general inventive concept.

The foregoing and/or other aspects and utilities of the present generalinventive concept may be achieved by providing an error concealmentmethod for an audio signal. The error concealment method includes whenan error occurs in a current frame, selecting one of an errorconcealment in a frequency domain and an error concealment in a timedomain as an error concealment scheme for the current frame based on apredetermined criteria, when the error concealment in the frequencydomain is selected, selecting one of a repetition scheme and aninterpolation scheme in the frequency domain as the error concealmentscheme for the current frame based on a predetermined criteria, andconcealing the error of the current frame using the selected scheme.

The selecting one of the error concealment operation may includeselecting one of the error concealment in the frequency domain and theerror concealment in the time domain as the error concealment scheme forthe current frame based on a least one of error occurrence conditionsand window types of a previous frame preceding the current frame and anext frame following the current frame.

The selecting one of the error concealment operation may include, whenat least one of the previous frame and the next frame is a good frame,selecting one of the error concealment in the frequency domain and theerror concealment in the time domain as the error concealment scheme forthe current frame based on the window type of the good frame.

The selecting one of the error concealment operation may include, whenboth a previous frame preceding the current frame and a next framefollowing the current frame are good frames, selecting one of the errorconcealment in the frequency domain and the error concealment in thetime domain as the error concealment scheme for the current frame basedon an error concealment scheme used to conceal an error occurring in theprevious frame.

The selecting one of the error concealment operation may includeselecting one of the error concealment in the frequency domain and theerror concealment in the time domain as the error concealment scheme forthe current frame based on the length of a frame interval including theerror.

The concealing operation may include, when the interpolation scheme isselected in the selecting one of the repetition scheme operation,concealing the error of the current frame according to the interpolationscheme using an amplitude of the spectrum of one of a previous goodframe and a next good frame and a sign of the spectrum of the otherframe.

The concealing operation may include, when the interpolation scheme isselected in the selecting one of the repetition scheme operation,concealing the error of the current frame according to the interpolationscheme for every scale factor band using the amplitude of the spectrumof a scale factor band of one of a previous good frame and a next goodframe and a sign of the spectrum of the other scale factor band.

The concealing operation may include, when the interpolation scheme isselected in the selecting one of the repetition scheme operation,concealing the error of the current frame according to the interpolationscheme for every frequency bin using the amplitude of one of thespectrums of a previous good frame and a next good frame and a sign ofthe other spectrum.

The concealing operation may include, when the interpolation scheme isselected in the selecting one of the repetition scheme operation,concealing the error of the current frame according to the interpolationscheme using the spectrum of one of a previous good frame and a nextgood frame, which is selected based on the energies of the previous goodframe and the next good frame.

The concealing operation may include, when the interpolation scheme isselected in the selecting one of the repetition scheme operation,concealing the error of the current frame according to the interpolationscheme for every scale factor band using the spectrum of a scale factorband of one of a previous good frame and a next good frame, which isselected based on the energies of the scale factor bands of the previousgood frame and the next good frame.

The concealing operation may include, when the repetition scheme isselected in the selecting one of the repetition scheme operation,concealing the error of the current frame according to the repetitionscheme using the spectrum of a good frame having a window type that is along window, which is selected from among a previous frame preceding thecurrent frame and a next frame following the current frame.

The concealing operation may include, when the error concealment in thetime domain is selected in the selecting one of the error concealmentoperation, if at least one of a previous frame preceding the currentframe and a next frame following the current frame is a good frame,concealing the error of the current frame according to the errorconcealment in the time domain using the at least one good frame.

The error concealment method may further include applying fade-in andfade-out operations to an audio signal of the current frame that hasbeen error-concealed in the frequency domain or in the time domain.

The error concealment method may further include determining whether theerror occurs in the current frame.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing an error concealmentapparatus for an audio signal. The error concealment apparatus includesa first selection unit, a second selection unit, and an errorconcealment unit. When an error occurs in a current frame, the firstselection unit selects one of an error concealment in a frequency domainand an error concealment in a time domain as an error concealment schemefor the current frame based on a predetermined criteria. When the errorconcealment in the frequency domain is selected, the second selectionunit selects one of a repetition scheme and an interpolation scheme inthe frequency domain as the error concealment scheme for the currentframe based on predetermined criteria. The error concealment unitconceals the error of the current frame using the selected scheme.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing a decoding methodfor an audio signal. The decoding method includes determining whether anerror occurs in a current frame, decoding the current frame when theerror does not occur in the current frame, selecting one of an errorconcealment in a frequency domain and an error concealment in a timedomain as an error concealment scheme for the current frame based on apredetermined criteria when the error occurs in the current frame,selecting one of a repetition scheme and an interpolation scheme in thefrequency domain as the error concealment scheme for the current framebased on a predetermined criteria when the error concealment in thefrequency domain is selected, and concealing the error of the currentframe using the selected scheme.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing a decoding apparatusfor an audio signal. The decoding apparatus includes an error detectionunit that determines whether an error occurs in a current frame, adecoding unit that decodes the current frame when the error does notoccur in the current frame, a first selection unit that selects one ofan error concealment in a frequency domain and an error concealment in atime domain as an error concealment scheme for the current frame based aon predetermined criteria when the error occurs in the current frame, asecond selection unit that selects one of a repetition scheme and aninterpolation scheme in the frequency domain as the error concealmentscheme for the current frame based on a predetermined criteria when theerror concealment in the frequency domain is selected, and an errorconcealment unit that conceals the error of the current frame using theselected scheme.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing a computer-readablemedium having recorded thereon a program to implement one of the errorconcealment method and the decoding method.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing an error concealmentmethod for an audio signal transmitted by a plurality of frames, themethod including determining whether an error occurs in one of theplurality of frames and, if so, selecting one of an error concealmentscheme in a frequency domain and an error concealment scheme in a timedomain for the one frame based on predetermined criteria, and concealingthe error of the one frame using the selected error concealment scheme.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing a method toreconstruct an audio signal, the method including detecting whether anerror in a current frame exists, reconstructing the audio signal of thecurrent frame by performing an inverse modulated discrete cosinetransformation (MDCT) on a spectrum of the current frame, when thedetecting operation detects that the error in the current frame does notexist, identifying whether a position of the error is before a thresholdposition when the detecting operation detects that the error in thecurrent frame exists, reconstructing the audio signal of the currentframe by performing inverse MDCT on the spectrum of the current framewhen the identifying operation identifies that the position of the erroris not before the threshold position, determining whether the error canbe concealed in a frequency domain, when the identifying operationidentifies that the position of the error is before the thresholdposition, and reconstructing the audio signal of the current frame byconcealing the error in a time domain, when the determining operationdetermines that the error can not be concealed in the frequency domain.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing an audio processingsystem including an encoder having an error concealing apparatus toselect one of an error concealment in a frequency domain and an errorconcealment in a time domain as an error concealment scheme for thecurrent frame based on a predetermined criteria when an error occurs ina current frame, to select one of a repetition scheme and aninterpolation scheme in the frequency domain as the error concealmentscheme for the current frame based on a predetermined criteria when theerror concealment in the frequency domain is selected, and to concealthe error of the current frame using the selected scheme to generate anencoded audio signal and a decoder to decode the encoded audio signalaccording to information on the selected one of the error concealment inthe frequency domain and the error concealment in the time domain.

The foregoing and/or other aspects and utilities of the present generalinventive concept may also be achieved by providing a method ofoperating an audio processing system, the method including selecting oneof an error concealment in a frequency domain and an error concealmentin a time domain as an error concealment scheme for the current framebased on a predetermined criteria when an error occurs in a currentframe, selecting one of a repetition scheme and an interpolation schemein the frequency domain as the error concealment scheme for the currentframe based on a predetermined criteria when the error concealment inthe frequency domain is selected, and concealing the error of thecurrent frame using the selected scheme to generate an encoded audiosignal, and decoding the encoded audio signal according to informationon the selected one of the error concealment in the frequency domain andthe error concealment in the time domain.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and utilities of the present generalinventive concept will become apparent and more readily appreciated fromthe following description of the embodiments, taken in conjunction withthe accompanying drawings of which:

FIGS. 1A and 1B are flowcharts illustrating a decoding method for anaudio signal, which uses an error concealment method for an audio signalaccording to an exemplary embodiment of the present general inventiveconcept;

FIG. 2 illustrates a table illustrating error concealment schemes for acurrent frame according to error occurrence conditions and window typesof a previous frame and a next frame according to an embodiment of thegeneral inventive concept;

FIGS. 3A through 8B are detailed flowcharts illustrating a spectrumreconstruction operation using an interpolation method illustrated inFIGS. 1A and 1B according to exemplary embodiments of the presentgeneral inventive concept;

FIG. 9 is a block diagram illustrating a decoding apparatus for an audiosignal including an error concealment apparatus for an audio signalaccording to an exemplary embodiment of the present general inventiveconcept.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentgeneral inventive concept, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals refer to the likeelements throughout. The embodiments are described below in order toexplain the present general inventive concept by referring to thefigures.

FIGS. 1A and 1B are flowcharts illustrating a decoding method for anaudio signal, which uses an error concealment method for an audio signalaccording to an exemplary embodiment of the present general inventiveconcept.

Whether an error occurs in the current frame is checked in operation100. If an error does not occur in the current frame, i.e., the currentframe is a good frame, an inverse MDCT (modulated discrete cosinetransformation) is performed on spectrum coefficients of the currentframe, thereby reconstructing an audio signal of the current frame inoperation 110.

If an error occurs in the current frame, it is checked if an errorposition at which the error occurs is before or after a predeterminedthreshold position in operation 120. If the error position is after thethreshold position, the process goes to operation 110 to normally decodethe current frame. When a bitstream is composed of a plurality of layerslike a bitstream encoded using bit sliced arithmetic coding (BSAC),layers preceding the error position can be decoded normally. Thus, whenan error occurs in the last layer, which hardly has an influence uponthe sound quality of the audio signal, layers preceding the last layerwhere the error occurs are reconstructed normally and spectruminformation of the last layer can be reconstructed from spectruminformation of the reconstructed layers. Thus, in spite of thedetermination that the error occurs in the current frame in operation100, when the error position is after the threshold position, and thusthe error hardly has an influence upon the sound quality of the audiosignal, the error of the current frame is not concealed and the currentframe is decoded normally, instead of reconstructing the current framehaving the error using error concealment, thereby making the most ofinformation in layers where the error does not exist. A decoding resultfor a good frame that is normally decoded in operation 110 may be storedto be used in error concealment for a next frame.

If it is determined that the error position is before the thresholdposition in operation 120, another determination is made as to whetherthe error of the current frame can be concealed in a frequency domain,i.e., whether the error concealment in the frequency domain isapplicable to conceal the error of the current frame, based on erroroccurrence conditions and window types of a previous frame preceding thecurrent frame and a next frame following the current frame in operation130. Detailed criteria for the determination in operation 130 will bedescribed below.

When it is determined that the error of the current frame cannot beconcealed in the frequency domain in operation 130, the error of thecurrent frame is concealed in a time domain in operation 140. When it isdetermined that the error of the current frame can be concealed in thefrequency domain in operation 130, it is determined whether to concealthe error in the frequency domain or in the time domain based on alength of a frame interval including the error in operation 150. Morespecifically, when the length of the frame interval including the errorexceeds a predetermined threshold length, it is determined to concealthe error in the time domain. When the length of the frame intervalincluding the error is less than the threshold length, it is determinedto conceal the error in the frequency domain. This is because, althoughan error occurring over a long interval is concealed using aninterpolation scheme, the length of an error-concealed interval duringwhich the error is concealed increases due to the repetition scheme andthus the same spectrum is repeated over the long interval, causingmodulation noise. Accordingly, when the error occurs over a longinterval, error concealment in the time domain is performed even iferror concealment in the frequency domain is available, therebyminimizing modulation noise and sound quality degradation.

When it is determined to conceal the error in the time domain inoperation 150, the process goes to operation 140 to conceal the error ofthe current frame in the time domain. In error concealment in the timedomain, the audio signal of the current frame is reconstructed using anaudio signal in a time domain of a previous good frame (PGF) and/or anaudio signal in a time domain of a next good frame (NGF). When it isdetermined to conceal the error in the frequency domain in operation150, it is determined whether to use the repetition scheme in thefrequency domain or the interpolation scheme in the frequency domain toconceal the error of the current frame based on error occurrenceconditions and window types of the previous frame and the next frame inoperation 160. As mentioned above, when the length of theerror-concealed interval using the repetition scheme increases,modulation noise may be generated. Thus, it may be desirable to use theinterpolation scheme when the interpolation scheme is applicable and usethe repetition scheme only when the interpolation scheme is notapplicable based on error occurrence conditions and window types of theprevious frame and the next frame. Criteria for the determination ofwhether the interpolation scheme is applicable in operation 160 will bedescribed below.

When it is determined that the interpolation scheme is applicable toconceal the error of the current frame in operation 160, the error ofthe current frame is concealed using the interpolation scheme and thespectrum of the previous frame and the spectrum of the next frame inoperation 170. When it is determined that the interpolation scheme isnot applicable in operation 160, the error of the current frame isconcealed using the repetition scheme in operation 180. The spectrum ofthe current frame is reconstructed using the spectrum of the PGF and thespectrum of the NGF in the interpolation scheme and using the spectrumof one of the PGF and the NGF in the repetition scheme.

In operation 190, an inverse MDCT is performed on the spectrum of thecurrent frame, which has been reconstructed in operation 170 oroperation 180, thereby reconstructing the audio signal of the currentframe. In operation 195, fade-in and fade-out post-processing operationsin the time domain are performed on the audio signal of the currentframe, which has been reconstructed in operation 140 or operation 190,to allow the reconstructed audio signal to be heard naturally.

Hereinafter, the criteria for the determination of whether errorconcealment in the frequency domain is applicable in operation 130 andthe criteria for the determination of whether the interpolation schemein the frequency domain is applicable in operation 160 will be describedin detail. According to an embodiment of the present general inventiveconcept, one of error concealment in the frequency domain and errorconcealment in the time domain is selected to conceal the error of thecurrent frame based on the error occurrence conditions and window typesof the previous frame and the next frame in operation 130, and one ofthe repetition scheme and the interpolation scheme in the frequencydomain is selected to conceal the error of the current frame based onthe error occurrence conditions and window types of the previous frameand the next frame in operation 160.

Accordingly, the error occurrence condition of a frame indicates whetheror not an error occurs in the frame. Thus, a frame where an error occursis marked with E (error) and a frame where an error does not occur ismarked with G (good). The window type of a frame indicates whether thewindow of the frame is a long window or a short window. Thus, a framehaving a long window is marked with L (long) and a frame having a shortwindow is marked with S (short). Although the window type may furtherinclude a long stop window that is inserted when a change from the longwindow to the short window exists and a long start window that isinserted when a change from the short window to the long window exists,the other windows except for the long window will be referred to asshort windows for convenience of explanation. In general, the longwindow is used for modulated discrete cosine transformation (MDCT) withrespect to a stationary signal that hardly changes during a longinterval and is stable, and the short window is used for MDCT withrespect to a transient signal that sharply changes or includes a suddenattack signal.

The following principles are used as the criteria for the determinationof an error concealment scheme for the current frame in operation 130and operation 160.

In order to apply error concealment in the frequency domain to the errorof the current frame, at least one of the previous frame and the nextframe has to be a good frame. This is because the interpolation schemereconstructs the spectrum of the current frame by interpolating thespectrum of the PGF and the spectrum of the NGF and the repetitionscheme reconstructs the spectrum of the current frame by copying thespectrum of one of the PGF and the NGF.

The window type of a good frame used in error concealment in thefrequency domain has to be a long window. This is because, in theinterpolation scheme, when the window type of the PGF and the windowtype of the NGF are different from each other, spectrum coefficients inthe frequency domain cannot be interpolated, due to the nature of MDCT.Also in the repetition scheme, when the window type of a good frame tobe copied to the current frame is not a long window, the audio signal ofthe current frame, which is reconstructed by copying the spectrum of thegood frame to the current frame, may cause unexpected noise. Forexample, for the PGF undergoing MDCT using 8 short windows, when thespectrum of the current frame is reconstructed by repeating spectrumdata corresponding to the last window of the PGF 8 times and the lastwindow includes an attack signal or a noise signal, the attack signal orthe noise signal may be repetitively output from the current frame.Moreover, since the spectrum data corresponding to the last window isrepeated 8 times, modulation noise may be generated.

When the previous frame is an error frame and the error of the previousframe is concealed in the time domain, the spectrum of the previousframe is not supposed to be reconstructed. Accordingly, since thereconstructed spectrum of the previous frame cannot be used to concealthe error of the current frame, the current frame may be reconstructedusing a next frame when the next frame is a good frame, or the error ofthe current frame may be concealed in the time domain using areconstructed time domain signal of the previous frame. Thus, therepetition scheme that uses the spectrum of the previous frame cannot beused to conceal the error of the current frame.

Hereinafter, error concealment schemes to conceal the error of thecurrent frame, which are determined based on the error occurrenceconditions and window types of the previous frame and the next frame,according to an exemplary embodiment of the present general inventiveconcept will be described with reference to FIG. 2. FIG. 2 illustrates atable illustrating error concealment schemes for the current frameaccording to the error occurrence conditions and window types of theprevious frame and the next frame.

For example, where both the previous frame and the next frame are goodframes will be considered. When the window types of the PGF and the NGFare long widows, the interpolation scheme in the frequency domain can beapplied and thus the error of the current frame is concealed using theinterpolation scheme. When the window type of only one of the PGF andthe NGF is a long window, the error of the current frame is concealed byrepeating the spectrum of a frame that is MDCT-transformed using a longwindow in the current frame. When the window type of neither the PGF northe NGF is a long window, the error of the current frame is concealedusing error concealment in the time domain. Accordingly, errorconcealment in the frequency domain, such as the interpolation scheme orthe repetition scheme, cannot be used as discussed above. Instead, itmay be desirable to reconstruct a time-domain signal of the currentframe using both a time-domain signal of the PGF and a time-domainsignal of the NGF.

Alternatively, where only one of the previous frame and the next frameis a good frame and the other is an error frame will be considered.Also, whether or not the window type of the good frame is a long windowwill be considered. When the window type of the PGF or the NGF is a longwindow, the error of the current frame is concealed according to therepetition scheme using the good frame having the long window.Accordingly, when the previous frame is an error frame and the windowtype of the NGF is a long window, the error of the current frame may beconcealed according to the repetition scheme using the NGF. However, ifthe error of the previous frame has not been concealed in the timedomain, i.e., the error of the previous frame has been concealed usingthe repetition scheme, the spectrum of the current frame may bereconstructed by interpolating the reconstructed spectrum of theprevious frame and the spectrum of the NGF, thereby concealing the errorof the current frame. When the window type of the PGF or the NGF is nota long window, the repetition scheme cannot be used and thus the errorof the current frame is concealed using error concealment in the timedomain. Accordingly, an error concealment scheme for the current frameis determined in a similar manner as to where the window type of onlyone of the PGF and the NGF is a long window or where the window type ofneither the PGF nor the NGF is a long window.

Accordingly, where both the previous frame and the next frame are errorframes will be considered. An error concealment scheme that can beapplied to the error of the current frame is determined according to anerror concealment scheme used to conceal the error of the previousframe. Where the error of the previous frame is concealed using therepetition scheme in the frequency domain will be discussed. Since thecurrent frame is an error frame, the interpolation scheme may not beused in the previous frame. When the error of the previous frame isconcealed using the repetition scheme, the error of the current framemay also be concealed by repeating a spectrum that has been repeated forthe previous frame. However, when modulation noise is likely to begenerated because a frame error has been concealed using the repetitionscheme more than a consecutive number of times, the error concealmentscheme may be changed to conceal the frame error using the time domain.Alternatively, where the error of the previous frame is concealed in thetime domain will be discussed. Since the error of the previous frame isconcealed in the time domain, there is no reconstructed spectrum of theprevious frame. Accordingly, the spectrum of the previous frame cannotbe used to conceal the error of the current frame using the repetitionscheme. Thus, when the error of the previous frame is concealed usingerror concealment in the time domain, the error of the current frame mayalso be concealed using error concealment in the time domain.

Thus, in operation 130 (FIG.1A), whether error concealment in thefrequency domain, i.e., the repetition scheme or the interpolationscheme, is applicable to conceal the error of the current frame can bedetermined as follows: it is determined that error concealment in thefrequency domain can be applied to conceal the error of the currentframe (1) where at least one of the previous frame and the next frame isa good frame and at least one of the window types of good frames is along window or (2) where neither the previous frame nor the next frameare good frames, i.e., both the previous frame and the next frame areerror frames, and error concealment in the frequency domain, i.e., therepetition scheme, is applied to conceal the error of the previousframe, and otherwise, it is determined that error concealment in thefrequency domain cannot be applied and thus the error of the currentframe is concealed using error concealment in the time domain.

Also, in operation 160 (FIG. 1B), whether to use the interpolationscheme in the frequency domain or the repetition scheme in the frequencydomain to conceal the error of the current frame can be determined asfollows: it is determined to use the interpolation scheme to conceal theerror of the frame (1) where both the previous frame and the next frameare good frames and the window types of both the previous frame and thenext frame are long windows or (2) where the error of the previous framehas been concealed using the repetition scheme, the next frame is a goodframe, and the window type of the next frame is a long window, and it isdetermined to use the repetition scheme to conceal the error of thecurrent frame such as (3) where both the previous frame and the nextframe are good frames, but the window type of only one of the previousframe and the next frame is a long window, (4) where only one of theprevious frame and the next frame is a good frame and the window type ofthe good frame is a long window, or (5) where both the previous frameand the next frame are error frames and the error of the previous frameis concealed using the repetition scheme.

As discussed above, however, when the previous frame is an error frame,the next frame is a good frame, and the window type of the next frame isa long window, if the error of the previous frame has been concealedusing the repetition scheme, the error of the current frame can beconcealed by interpolating the reconstructed spectrum of the previousframe and the spectrum of the next good frame or by repeating thespectrum of the next good frame. Thus, one of the interpolation schemeand the repetition scheme may be determined as an error concealmentscheme for the current frame.

Thus, according to an embodiment of the present general inventiveconcept, the optimal error concealment scheme for the current frame canbe selected from among the repetition scheme and the interpolationscheme in the frequency domain and error concealment in the time domainbased on the error occurrence conditions and window types of theprevious frame and the next frame and the error of the current frame canbe concealed using the selected scheme, thereby minimizing modulationnoise and sound quality degradation in the reconstructed audio signal.

Hereinafter, error concealment using the interpolation scheme inoperation 170 according to exemplary embodiments of the present generalinventive concept will be described with reference to FIGS. 3A through8B. FIGS. 3A through 8B are detailed flowcharts illustrating operation170 (FIG. 1B) according to exemplary embodiments of the present generalinventive concept.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 3A, for every frame, the spectrum of thecurrent frame is generated by combining the amplitude of the spectrum ofthe PGF and the sign of the spectrum of the NGF, thereby concealing theerror of the current frame. Accordingly, the amplitude of the spectrumof the PGF is extracted in operation 300 and the sign of the spectrum ofthe NGF is extracted in operation 310. The spectrum of the current frameis reconstructed by combining the extracted spectrum data in operation320.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 3B, for every frame, the spectrum of thecurrent frame is generated by combining the sign of the spectrum of thePGF and the amplitude of the spectrum of the NGF, thereby concealing theerror of the current frame. Accordingly, the sign of the spectrum of thePGF is extracted in operation 330 and the amplitude of the spectrum ofthe NGF is extracted in operation 340. In operation 350, the spectrum ofthe current frame is reconstructed by combining the extracted spectrumdata.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 4A, for every scale factor band (SFB), theamplitude of the spectrum of the SFB of one of the PGF and the NGF,which has the greater energy than the SFB of the other, and the sign ofthe spectrum of the SFB of the other are combined, therebyreconstructing the spectrum of the current frame. More specifically, forevery SFB, the energy of the SFB of the PGF and the energy of the SFB ofthe NGF are calculated in operation 400. The amplitude of the spectrumof the SFB having the greater energy between the energy of the SFB ofthe PGF and the energy of the SFB of the NGF is extracted in operation410. The sign of the spectrum of the SFB having the smaller energy isextracted in operation 420. The extracted amplitude of the spectrum andthe extracted sign of the other spectrum are combined, therebyreconstructing the spectrum of the current frame in operation 430.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 4B, for every SFB, the sign of the spectrumof the SFB of one of the PGF and the NGF, which has the greater energythan the SFB of the other, and the amplitude of the spectrum of the SFBof the other are combined, thereby reconstructing the spectrum of thecurrent frame. More specifically, for every SFB, the energy of the SFBof the PGF and the energy of the SFB of the NGF are calculated inoperation 440. The sign of the spectrum of the SFB having the greaterenergy between the energy of the SFB of the PGF and the energy of theSFB of the NGF is extracted in operation 450. The amplitude of thespectrum of the SFB having the smaller energy is extracted in operation460. The extracted sign of the spectrum and the extracted amplitude ofthe other spectrum are combined, thereby reconstructing the spectrum ofthe current frame in operation 470.

Alternatively, for every SFB, one of the SFB of the PGF and the SFB ofthe NGF may be selected at random and the amplitude of the spectrum ofthe selected SFB and the sign of the spectrum of the other SFB may becombined in order to reconstruct the spectrum of the current frame.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 5A, for every frequency bin, the amplitudeof the spectrum of one of the PGF and the NGF, which has the greaterenergy than the spectrum of the other, and the sign of the spectrum ofthe other are combined, thereby reconstructing the spectrum of thecurrent frame. More specifically, for every frequency bin, the energy ofthe spectrum of the PGF and the energy of the spectrum of the NGF arecalculated in operation 500. The amplitude of the spectrum having thegreater energy between the energy of the spectrum of the PGF and theenergy of the spectrum of the NGF is extracted in operation 510 and thesign of the spectrum having the smaller energy is extracted in operation520. The extracted amplitude of the spectrum and the extracted sign ofthe other spectrum are combined, thereby reconstructing the spectrum ofthe current frame in operation 530.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 5B, for every frequency bin, the sign of thespectrum of one of the PGF and the NGF, which has the greater energythan the spectrum of the other, and the amplitude of the spectrum of theother are combined, thereby reconstructing the spectrum of the currentframe. More specifically, for every frequency bin, the energy of thespectrum of the PGF and the energy of the spectrum of the NGF arecalculated in operation 540. The sign of the spectrum having the greaterenergy between the energy of the spectrum of the PGF and the energy ofthe spectrum of the NGF is extracted in operation 550 and the amplitudeof the spectrum having the smaller energy is extracted in operation 560.The extracted sign of the spectrum and the extracted amplitude of theother spectrum are combined, thereby reconstructing the spectrum of thecurrent frame in operation 570.

Alternatively, for every frequency bin, one of the spectrum of the PGFand the spectrum of the NGF may be selected at random and the amplitudeof the selected spectrum and the sign of the other spectrum may becombined in order to reconstruct the spectrum of the current frame.

In the exemplary embodiments illustrated in FIGS. 3A through 5B, thespectrum of the current frame is reconstructed by combining theamplitude of the spectrum of one of the PGF and the NGF and the sign ofthe spectrum of the other, i.e., using spectrum composition. However, inthe following exemplary embodiments illustrated in FIGS. 6A through 8B,the spectrum of the current frame is reconstructed by referring to boththe amplitude and the sign of the spectrum of one of the PGF and theNGF, i.e., using spectrum merging.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 6A, the spectrum of the current frame isreconstructed using the spectrum of one of the PGF and the NGF, whichhas the greater energy than the other frame. More specifically, theenergy of the PGF and the energy of the NGF are calculated in operation600. The amplitude and sign of the spectrum of one of the PGF and theNGF, which has the greater energy than the other frame, are extracted inoperation 610. In operation 620, the spectrum of the current frame isreconstructed using the extracted amplitude and sign.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 6B, the spectrum of the current frame isreconstructed using the spectrum of one of the PGF and the NGF, whichhas the smaller energy than the other frame. More specifically, theenergy of the PGF and the energy of the NGF are calculated in operation630. The amplitude and sign of the spectrum of one of the PGF and theNGF, which has the smaller energy than the other frame, are extracted inoperation 640. In operation 650, the spectrum of the current frame isreconstructed using the extracted amplitude and sign.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 7A, for every SFB, the spectrum of thecurrent frame is reconstructed using the amplitude and sign of thespectrum of the SFB of one of the PGF and the NGF, which has the greaterenergy than the SFB of the other. More specifically, for every SFB, theenergy of the SFB of the PGF and the energy of the SFB of the NGF arecalculated in operation 700. The amplitude and sign of the spectrum ofthe SFB having the greater energy between the energy of the SFB of thePGF and the energy of the SFB of the NGF are extracted in operation 710.The spectrum of the current frame is reconstructed using the extractedamplitude and sign in operation 720.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 7B, for every SFB, the spectrum of thecurrent frame is reconstructed using the amplitude and sign of thespectrum of the SFB of one of the PGF and the NGF, which has the smallerenergy than the SFB of the other. More specifically, for every SFB, theenergy of the SFB of the PGF and the energy of the SFB of the NGF arecalculated in operation 730. The amplitude and sign of the spectrum ofthe SFB having the smaller energy between the energy of the SFB of thePGF and the energy of the SFB of the NGF are extracted in operation 740.The spectrum of the current frame is reconstructed using the extractedamplitude and sign in operation 750.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 8A, for every frequency bin, the spectrum ofthe current frame is reconstructed using the amplitude and sign of thespectrum of one of the PGF and the NGF, which has the greater energythan the spectrum of the other. More specifically, for every frequencybin, the energy of the spectrum of the PGF and the energy of thespectrum of the NGF are calculated in operation 800. The amplitude andsign of the spectrum having the greater energy between the energy of thespectrum of the PGF and the energy of the spectrum of the NGF areextracted in operation 810. In operation 820, the spectrum of thecurrent frame is reconstructed using the extracted amplitude and signfor every frequency bin.

According to an exemplary embodiment of the present general inventiveconcept illustrated in FIG. 8B, for every frequency bin, the spectrum ofthe current frame is reconstructed using the amplitude and sign of thespectrum of one of the PGF and the NGF, which has the smaller energythan the spectrum of the other. More specifically, for every frequencybin, the energy of the spectrum of the PGF and the energy of thespectrum of the NGF are calculated in operation 830. The amplitude andsign of the spectrum having the smaller energy between the energy of thespectrum of the PGF and the energy of the spectrum of the NGF areextracted in operation 840. In operation 850, the spectrum of thecurrent frame is reconstructed using the extracted amplitude and signfor every frequency bin.

FIG. 9 is a block diagram of a decoding apparatus for an audio signalincluding an error concealment apparatus 920 for an audio signalaccording to an exemplary embodiment of the present general inventiveconcept. The decoding apparatus includes an error detection unit 900, adecoding unit 910, the error concealment apparatus 920, and apost-processing unit 930. The error concealment apparatus 920 includes afirst selection unit 940, a second selection unit 950, and an errorconcealment unit 960. Hereinafter, the operation of the errorconcealment apparatus 920 and the operation of the decoding apparatusincluding the error concealment apparatus 920 according to an exemplaryembodiment of the present general inventive concept will be describedwith reference to FIG. 9.

The error detection unit 900 checks if an error occurs in the currentframe. If an error occurs in the current frame, the error detection unit900 checks if an error position at which the error occurs is before orafter a predetermined threshold position. If the error does not occur inthe current frame, i.e., the current frame is a good frame, or if theerror occurs and the error position is after the threshold position, theerror detection unit 900 transmits the current frame to the decodingunit 910. If the error occurs in the current frame and the errorposition is before the threshold position, the error detection unit 900transmits the current frame to the error concealment apparatus 920. Whena bitstream is composed of a plurality of layers like a bitstreamencoded using bit sliced arithmetic coding (BSAC), layers preceding theerror position can be decoded normally. Thus, when an error occurs inthe last layer, which hardly has an influence upon the sound quality ofthe audio signal, layers preceding the last layer where the error occursare reconstructed normally and spectrum information of the last layercan be reconstructed from spectrum information of the reconstructedlayers. Thus, although the determination made by the error detectionunit 900 that the error occurs in the current frame, when the errorposition is after the threshold position, and thus the error hardly hasan influence upon the sound quality of an audio signal, the error of thecurrent frame is not concealed. Also, the current frame is decodednormally, instead of reconstructing the current frame having the errorusing error concealment, thereby making the most of information inlayers where the error does not exist.

The decoding unit 910 normally decodes the current frame transmittedfrom the error detection unit 900 and the error concealment apparatus920 conceals the error of the current frame transmitted from the errordetection unit 900. A decoding result for a good frame may be stored tobe used in error concealment for a next frame.

The first selection unit 940 determines whether the error of the currentframe can be concealed in the frequency domain, i.e., error concealmentin the frequency domain is applicable to conceal the error of thecurrent frame, based on error occurrence conditions and window types ofa previous frame preceding the current frame and a next frame followingthe current frame. Detailed criteria for the determination performed bythe first selection unit 940 will be described below.

When the first selection unit 940 determines that the error of thecurrent frame can be concealed in the frequency domain, the firstselection unit 940 further determines whether to conceal the error inthe frequency domain or in the time domain based on the length of aframe interval including the error. More specifically, when the lengthof the frame interval including the error exceeds a predeterminedthreshold length, the first selection unit 940 determines to conceal theerror in the time domain. When the length of the frame intervalincluding the error does not exceed the threshold length, the firstselection unit 940 determines to conceal the error in the frequencydomain. This is because, although the error occurring over a longinterval is concealed using the interpolation scheme, the length of anerror-concealed interval during which the error is concealed increasesdue to the repetition scheme and thus the same spectrum is repeated overthe long interval, causing modulation noise. Accordingly, when the erroroccurs over a long interval, error concealment in the time domain isperformed even if error concealment in the frequency domain isavailable, thereby minimizing modulation noise and sound qualitydegradation.

The first selection unit 940 determines that the error of the currentframe can be concealed in the frequency domain and determines to concealthe error of the current frame in the frequency domain based on thelength of a frame interval including the error. The second selectionunit 950 determines whether to conceal the error of the current frameusing the repetition scheme in the frequency domain or the interpolationscheme in the frequency domain based on the error occurrence conditionsand window types of the previous frame and the next frame. As mentionedabove, when the length of the error-concealed interval using therepetition scheme increases, modulation noise may be generated. Thus, itmay be desirable to use the interpolation scheme when the interpolationscheme is applicable and use the repetition scheme only when theinterpolation scheme is not applicable based on error occurrenceconditions and window types of the previous frame and the next frame.Criteria for the determination performed by the second selection unit950 will be described below.

When the first selection unit 940 determines that the error of thecurrent frame can be concealed in the frequency domain and the length ofthe frame interval including the error does not exceed the thresholdlength, the error concealment unit 960 conceals the error of the currentframe using the interpolation scheme or the repetition scheme. Thus, theerror concealment unit 960 reconstructs the spectrum of the currentframe using the spectrum of the PGF and/or the spectrum of the NGF andperforms an inverse MDCT on the reconstructed spectrum, therebyreconstructing the audio signal of the current frame.

When the first selection unit 940 determines that the error of thecurrent frame cannot be concealed in the frequency domain or the lengthof the frame interval including the error exceeds the threshold lengtheven if the error of the current frame can be concealed in the frequencydomain, the error concealment unit 960 conceals the error of the currentframe in the time domain. In the time domain, the error concealment unit960 reconstructs the audio signal of the current frame sing atime-domain audio signal of the PGF and/or a time-domain audio signal ofthe NGF.

The post-processing unit 930 performs fade-in and fade-out operations inthe time domain on the audio signal of the current frame, which has beenreconstructed by the error concealment apparatus 920, in order to allowthe reconstructed audio signal to be heard naturally.

Hereinafter, the criteria for the determination performed by the firstselection unit 940 of whether error concealment in the frequency domainis applicable and the criteria for the determination performed by thesecond selection unit 950 of whether the interpolation scheme in thefrequency domain is applicable will be described in detail. According toan embodiment of the present general inventive concept, the firstselection unit 940 selects one of error concealment in the frequencydomain and error concealment in the time domain to conceal the error ofthe current frame based on the error occurrence conditions and windowtypes of the previous frame and the next frame. Also, the secondselection unit 950 selects one of the repetition scheme and theinterpolation scheme in the frequency domain to conceal the error of thecurrent frame based on the error occurrence conditions and window typesof the previous frame and the next frame.

Accordingly, the error occurrence condition of a frame indicates whetheror not an error occurs in the frame. Thus, a frame where an error occursis marked with E (error) and a frame where an error does not occur ismarked with G (good). The window type of a frame indicates whether thewindow of the frame is a long window or a short window. Thus, a framehaving a long window is marked with L (long) and a frame having a shortwindow is marked with S (short). Although the window type may furtherinclude a long stop window that is inserted when a change from the longwindow to the short window and a long start window that is inserted whena change from the short window to the long window, the other windowsexcept for the long window will be referred to as short windows forconvenience of explanation. In general, the long window is used formodulated discrete cosine transformation (MDCT) with respect to astationary signal that hardly changes during a long interval and isstable and the short window is used for MDCT with respect to a transientsignal that sharply changes or includes a sudden attack signal.

The following principles are used as the criteria for the determinationof an error concealment scheme for the current frame, which is performedby the first selection unit 940 and the second selection unit 950.

In order to apply error concealment in the frequency domain to the errorof the current frame, at least one of the previous frame and the nextframe has to be a good frame. This is because the interpolation schemereconstructs the spectrum of the current frame by interpolating thespectrum of the PGF and the spectrum of the NGF and the repetitionscheme reconstructs the spectrum of the current frame by copying thespectrum of one of the PGF and the NGF.

The window type of a good frame used in error concealment in thefrequency domain has to be a long window. This is because, in theinterpolation scheme, when the window type of the PGF and the windowtype of the NGF are different from each other, spectrum coefficients inthe frequency domain cannot be interpolated, due to the nature of MDCT.Also in the repetition scheme, when the window type of a good frame tobe copied to the current frame is not a long window, the audio signal ofthe current frame, which is reconstructed by copying the spectrum of thegood frame to the current frame, may cause unexpected noise. Forexample, for the PGF undergoing MDCT using 8 short windows, when thespectrum of the current frame is reconstructed by repeating spectrumdata corresponding to the last window of the PGF 8 times and the lastwindow includes an attack signal or a noise signal, the attack signal orthe noise signal may be repetitively output from the current frame.Moreover, since the spectrum data corresponding to the last window isrepeated 8 times, modulation noise may be generated.

When the previous frame is an error frame and the error of the previousframe is concealed in the time domain, the spectrum of the previousframe is not supposed to be reconstructed. Accordingly, since thereconstructed spectrum of the previous frame cannot be used to concealthe error of the current frame, the current frame may be reconstructedusing a next frame when the next frame is a good frame, or the error ofthe current frame may be concealed in the time domain using areconstructed time domain signal of the previous frame. Thus, therepetition scheme that uses the spectrum of the previous frame cannot beused to conceal the error of the current frame.

Hereinafter, error concealment schemes to control the error of thecurrent frame, which are determined based on the error occurrenceconditions and window types of the previous frame and the next frame,according to an exemplary embodiment of the present general inventiveconcept, will be described with reference to FIG. 2. FIG. 2 illustratesa table illustrating error concealment schemes for the current frameaccording to the error occurrence conditions and window types of theprevious frame and the next frame according to the present inventiveconcept.

Where both the previous frame and the next frame are good frames will beconsidered. When the window types of the PGF and the NGF are longwidows, the interpolation scheme in the frequency domain can be appliedand thus the error of the current frame is concealed using theinterpolation scheme. When the window type of only one of the PGF andthe NGF is a long window, the error of the current frame is concealed byrepeating the spectrum of a frame that is MDCT-transformed using a longwindow in the current frame. When the window type of neither the PGF northe NGF is a long window, the error of the current frame is concealedusing error concealment in the time domain. Accordingly, errorconcealment in the frequency domain, such as the interpolation scheme orthe repetition scheme, cannot be used as discussed above. Instead, itmay be desirable to reconstruct a time-domain signal of the currentframe using both a time-domain signal of the PGF and a time-domainsignal of the NGF.

Where only one of the previous frame and the next frame is a good frameand the other is an error frame will be considered. Further, whether ornot the window type of the good frame is a long window will beconsidered. When the window type of the PGF or the NGF is a long window,the error of the current frame is concealed according to the repetitionscheme using the good frame having the long window. Accordingly, whenthe previous frame is an error frame and the window type of the NGF is along window, the error of the current frame may be concealed accordingto the repetition scheme using the NGF. However, if the error of theprevious frame has not been concealed in the time domain, i.e., theerror of the previous frame has been concealed using the repetitionscheme, the spectrum of the current frame may be reconstructed byinterpolating the reconstructed spectrum of the previous frame and thespectrum of the NGF, thereby concealing the error of the current frame.When the window type of the PGF or the NGF is not a long window, therepetition scheme cannot be used and thus the error of the current frameis concealed using error concealment in the time domain. Accordingly, anerror concealment scheme for the current frame is determined in asimilar manner where the window type of only one of the PGF and the NGFis a long window or where the window type of neither the PGF nor the NGFis a long window.

Where both the previous frame and the next frame are error frames willbe considered. An error concealment scheme that can be applied to theerror of the current frame is determined according to an errorconcealment scheme used to conceal the error of the previous frame.Where the error of the previous frame is concealed using the repetitionscheme in the frequency domain will be discussed. Since the currentframe is an error frame, the interpolation scheme may not be used in theprevious frame. When the error of the previous frame is concealed usingthe repetition scheme, the error of the current frame may also beconcealed by repeating a spectrum that has been repeated for theprevious frame. However, when modulation noise is likely to be generatedbecause a frame error has been concealed using the repetition schemeover a consecutive number of times, the error concealment scheme may bechanged to conceal the frame error using the time domain. Alternatively,where the error of the previous frame is concealed in the time domainwill be discussed. Since, the error of the previous frame is concealedin the time domain, there is no reconstructed spectrum of the previousframe. Accordingly, the spectrum of the previous frame cannot be used toconceal the error of the current frame using the repetition scheme.Thus, when the error of the previous frame is concealed using errorconcealment in the time domain, the error of the current frame may alsobe concealed using error concealment in the time domain.

Thus, the first selection unit 940 determines whether error concealmentin the frequency domain, i.e., the repetition scheme or theinterpolation scheme, is applicable to conceal the error of the currentframe. Accordingly, the first selection unit 930 determines that errorconcealment in the frequency domain can be applied to conceal the errorof the current frame (1) where at least one of the previous frame andthe next frame is a good frame and at least one of the window types ofgood frames is a long window or (2) where neither the previous frame northe next frame are good frames, i.e., both the previous frame and thenext frame are error frames, and error concealment in the frequencydomain, i.e., the repetition scheme, is applied to conceal the error ofthe previous frame. Otherwise, the first selection unit 940 determinesthat error concealment in the frequency domain cannot be applied andthus the error of the current frame is concealed using error concealmentin the time domain.

Also, the second selection unit 950 determines whether to use theinterpolation scheme in the frequency domain or the repetition scheme inthe frequency domain to conceal the error of the current frame.Accordingly, the second selection unit 950 determines to use theinterpolation scheme to conceal the error of the frame (1) where boththe previous frame and the next frame are good frames and the windowtypes of both the previous frame and the next frame are long windows or(2) where the error of the previous frame has been concealed using therepetition scheme, the next frame is a good frame, and the window typeof the next frame is a long window. The second selection unit 950determines to use the repetition scheme to conceal the error of thecurrent frame such as (3) where both the previous frame and the nextframe are good frames, but the window type of only one of the previousframe and the next frame is a long window, (4) where only one of theprevious frame and the next frame is a good frame and the window type ofthe good frame is a long window, or (5) where both the previous frameand the next frame are error frames and the error of the previous frameis concealed using the repetition scheme.

As discussed above, however, when the previous frame is an error frame,the next frame is a good frame, and the window type of the next frame isa long window, if the error of the previous frame has been concealedusing the repetition scheme, the error of the current frame can beconcealed by interpolating the reconstructed spectrum of the previousframe and the spectrum of the next good frame or by repeating thespectrum of the next good frame. Thus, one of the interpolation schemeand the repetition scheme may be determined as an error concealmentscheme for the current frame.

Thus, according to an embodiment of the present general inventiveconcept, the optimal error concealment scheme for the current frame canbe selected from among the repetition scheme and the interpolationscheme in the frequency domain and error concealment in the time domainbased on the error occurrence conditions and window types of theprevious frame and the next frame. Also, the error of the current framecan be concealed using the selected scheme, thereby minimizingmodulation noise and sound quality degradation in the reconstructedaudio signal.

Meanwhile, the present general inventive concept can also be embodied ascomputer-readable code on a computer-readable medium. Thecomputer-readable medium is any data storage device that can store datathat can be thereafter read by a computer system. Examples of thecomputer-readable medium include read-only memory (ROM), random-accessmemory (RAM), CD-ROMs, magnetic tapes, floppy disks, and optical datastorage devices and carrier waves (such as data transmission through theInternet). The computer readable medium can also be distributed overnetwork-coupled computer systems so that the computer-readable code isstored and executed in a distributed fashion. Also, functional programs,codes and code segment to accomplish the present general inventiveconcept can be easily construed by programmers skilled in the art towhich the present general inventive concept pertains.

As described above, in the error concealment method and apparatus for anaudio signal according to the present general inventive concept, arepetition scheme and an interpolation scheme in a frequency domain areselectively utilized to conceal an error that occurs in a frame. Also,error concealment in a time domain is used when it is difficult orimpossible to conceal the error in the frequency domain, therebypreventing sound quality from degrading due to modulation noise.

Furthermore, in the decoding method and apparatus for an audio signalaccording to the present general inventive concept, sound quality can beprevented from degrading due to modulation noise by reconstructing anerror frame using the error concealment method and apparatus, therebyimproving perceptual sound quality.

Although a few embodiments of the present general inventive concept havebeen illustrated and described, it will be appreciated by those skilledin the art that changes may be made in these embodiments withoutdeparting from the principles and spirit of the general inventiveconcept, the scope of which is defined in the appended claims and theirequivalents.

What is claimed is:
 1. An apparatus for concealing an error in an audioinput signal, the apparatus comprising: at least one processorconfigured to: receive an audio input signal including a previous frameand a current frame, generate signal characteristics includingstationarity of the audio input signal, determine an error concealmentscheme of the current frame from a plurality of error concealmentschemes provided that the current frame corresponds to one of an errorframe and a good frame after an error frame, the error concealmentscheme being determined based on the signal characteristics and an errorconcealment scheme used for the previous frame, and conceal the currentframe based on the determined error concealment scheme.
 2. The apparatusof claim 1, wherein the signal characteristics further include aparameter obtained by using a length of the current frame.
 3. Theapparatus of claim 1, wherein the signal characteristics further includea parameter obtained by using a window type of the previous frame.
 4. Anapparatus for decoding an audio input signal, the apparatus comprising:at least one processor configured to: decode a first frame when no erroroccurs in the first frame; and determine an error concealment scheme ofa second frame from a plurality of error concealment schemes providedthat the second frame corresponds to a current error frame or a goodframe after an error frame, the error concealment scheme beingdetermined based on signal characteristics including stationarity of theaudio input signal and an error concealment scheme used for a previousframe of the second frame; and conceal the second frame based on thedetermined error concealment scheme.
 5. The apparatus of claim 4,wherein the signal characteristics further include a parameter obtainedby using a length of the second frame.
 6. The apparatus of claim 4,wherein the signal characteristics further include a parameter obtainedby using a window type of the previous frame.